A telephone line circuit, traditionally located at the telephone central office, provides a number of functions for the customer loop it serves. "BORSCHT" is the mnemonic frequently used to identify the principal ones of such functions: battery feed, over-voltage protection, ringing, supervision of the loop, coding/decoding, hybrid function, i.e., connection of two-wire subscriber line to the four-wire network, and testing.
The growing trend to the concentration of customer lines remote from the central office requires that BORSCHT functions be provided by line cards remotely located from the central office switch--with the result that the automatic testing equipment of the central office is no longer locally available to the line circuit. Accordingly, it would be desirable to provide a line interface circuit which could accomplish, in a self-contained manner, many of the testing functions priorly requiring the local availability of central office switching equipment. Additionally, it would be advantageous to reduce the cost and size of the line interface circuit without sacrificing reliability. In particular, it would be advantageous to eliminate the need for the line transformers and electromechanical loop isolation relay that has previously been required to disconnect the Tip and Ring conductors of the customer loop from the line circuit so the line circuit could be tested without being affected by, or affecting, conditions on the loop.
It has heretofore been recognized that one of the conventional electromechanical relays, namely, the ringing relay, whose transfer contacts connect the ring conductor of the loop either to the line circuit or to the ringing generator, may be replaced by a transistor switch. U.S. Pat. No. 4,652,701 issued Mar. 24, 1987 to R. J. Cubbison, Jr. teaches that when the relay is replaced by a transistor switch it is necessary to provide a feedback path to prevent the finite resistance of the cutoff transistor from coupling the ringing signal into the line circuit and to compensate for the transistor's finite forward resistance to prevent longitudinal unbalance.
It has also been appreciated, as for example in U.S. Pat. No. 5,341,416 issued Aug. 23, 1994 to J. C. Gammel, that the rapid detection of the off-hook condition when the ringing signal is applied to the loop, i.e., high-speed ring trip, can be achieved by monitoring the ringing signal polarity and cutting off the ringing current when it rises above a certain threshold amplitude during a particular polarity of the ringing voltage.
It would be advantageous if a line circuit could be provided which was capable, without need for an electromechanical loop isolating relay, of determining whether there are false crosses or grounds on the tip and ring conductors and of verifying the condition of the line circuit's fuses, DC feed and voice frequency terminating impedances, codec and tip and ring driving amplifiers as well as the ring trip and loop closure detectors. It would be advantageous to be able to ascertain the condition of these circuits under different battery supply voltages.